i would start out with them all the way up, and see if it runs hot.
if it isn't running hot, and it's noisy, you can turn them down a bit, and see if it improves the noise without starting to run hot.
for example, one of my case fans runs at 2700 rpms, and makes a considerable amount of noise.
now, it's moving alot of air, so i turn it down a bit, and i can go to about 2000 rpms, and not notice a single bit of difference in the temps, but, it suddenly gets a tons quieter.
now, i have 9 fans, and i absolutely had to do this to all my fans.
i call it noise tuning.